There are many theories about it's beginning. One consider that it was created by God. Others hold it as a result of difficult biochemical and evolutionary processes on the planet.
Some are sure that we come from other planets. Arrived from other solar system, probably even galaxies. Let's imagine that billions years ago is catastrophe of planetary scale. It served as the emission reason from the planet. Huge mass of various elementary organisms rich with vital forms in space which only and could survive there.
And these protozoans similar to seeds of a plant got on the planet suitable for development and evolution. This is how we live.